The currency being used by each country has different value when you use it in other countries. It means that not all currency has the same value and it somehow maintains balance towards each nation's economy although third world countries has the lowest value compared to other currencies. Bitcoin, as the world's only currency might be possible someday but there would be a lot of adjustments and innovation needed first so that it would work smoothly and properly regardless of the country. The outcome is still not clear since it is just a theory and there are not much proofs about it yet but maybe we'll know through El Salvador's recent legalization of bitcoin.